(a) Field of the Invention
The present invention relates to a method for determining an impurity concentration profile in a semiconductor integrated circuit.
(b) Description of the Related Art
Impurities are generally contained in a semiconductor in a very small amount as a dopant or contaminants, and have a great influence upon the functions of a semiconductor device. To illustrate this, the following description will be made with reference to a silicon device which is a typical device of a CMOS or DRAM.
In a silicon device, impurity ions are implanted into a silicon substrate as a dopant by an ion implantation process, then subjected to a thermal treatment for diffusion and activation of the dopant. It is known that a resultant impurity distribution causes a great change in electric parameters such as threshold voltage Vt or the like.
It is recognized in the art that when a cell size and accordingly gate length are reduced in an attempt to meet the demands for a higher density, a higher capacity and a higher speed of operation of silicon devices, there occurs a problem of the short channel effect which reduces the threshold voltage Vt of an FET. On the other hand, in order to simplify the process used, ions are implanted at various energy levels including a high energy range, and many processes are used in which a diffusion takes place at relatively low temperatures and within short intervals. As a consequence of these practices, it has become clear that an impurity distribution exhibits a very complicated behavior depending on residual damage caused by the ion implantation. The effect of such impurity distribution manifests itself as the reverse short channel effect in which the threshold voltage Vt increases in an extraordinary manner toward the short lengths of the gate, which becomes a critical factor in variation of the threshold voltage Vt, thereby presenting serious problems in the design of fine silicon devices.
To cope with such problems, it is known in the art that SIMS (Secondary Ion Spectrometry) process is effective as a method of determining the impurity distribution in the semiconductor. In the SIMS process, primary ions such as cesium or oxygen ions are irradiated to generate secondary ions, which are then subject to a mass spectroscopy. However, such process is only effective for a specimen in which the impurity is uniformly distributed over a sufficiently extensive region of the semiconductor.
SRP (Spread Resistance Profile) technique is also known in the prior art In which a specimen is polished along a skewed plane to determine a sheet resistance from the relationship between a voltage and a current across a pair of terminals. By this method, a known relationship between the sheet resistance and the impurity concentration, which is established experimentally, is utilized to derive a distribution of impurity concentrations within the semiconductor.
A chemical etching technique is also well known as a method of determining a two dimensional distribution of a junction. The method, however, can be used only when the impurity concentration is at a high level, and the accuracy thereof is not satisfactory.
In a semiconductor device, a trace amount of an impurity such as a dopant is distributed in a minute region in the depth direction as well as in lateral directions. It has been difficult to determine the distribution profile for a trace amount of impurities in a minute region of a micron order by using the prior art method mentioned above, due to the limitation of counting secondary ions in the SIMS process or the limitation of terminal size in the SRP process. While the chemical etching technique is simple and convenient in determining a two dimensional distribution, it requires the presence of sufficiently high impurity concentration, as mentioned above. In addition, the difficulty in controlling the chemical reaction results in an insufficient resolution in determining the depthwise or lateral spread.
A method of determining a lateral spread of impurities which are introduced by an ion implantation technique is disclosed in Proceeding of the 4th Conf. on Solid State Devices, Supplement to the Journal of the Japan Soc. of Appl. Phys., Vol. 42, 1973. This method is directed to determination of the profile in the depth direction while changing the angle of incidence of the ion implantation, and deriving the width of the lateral spread from the results thus obtained. In other words, this method is available only when the depthwise and lateral spread is previously known as functions of the incident angle of implantation.
A prior art knowledge in which a lateral spread and depthwise spread are determined as functions of the incident angle of implantation is available only for very restricted cases of ion implantation to which an analytical LSS theory is applicable, and accordingly this method cannot be applied in determining the profile in a semiconductor device subsequent to a process such as a thermal diffusion process.
Thus, it has been substantially impossible to determine an impurity profile in a semiconductor region which has sides on the order of sub-micron by using the prior art method mentioned above, although it is requested in fine semiconductor devices.